There is an ongoing need to analyze biological analytes accurately, quickly, and at reasonable cost. Indeed, the extent to which this can be done is one measure of a health care system's ability to provide satisfactory health care. An improvement in the ability to detect biomarkers would be beneficial in a variety of medical endeavors, such as the detection of cancer and other diseases.
A variety of techniques are currently used to detect analytes, in which analytical chemistry methods are employed to identify specific compounds of interest to a medical practitioner. An immunoassay is a biochemical test used to detect or measure the concentration of a chemical compound in a solution; it relies on the ability of antigens and antibodies to bind to each other with a high degree of specificity. Immunoassays can be employed to detect either the antigen or its corresponding antibody. One kind of immunoassay is the magnetic immunoassay, in which antigens and antibodies are bound to each other, and magnetic particles are then attached to the antigens (or antibodies) of the antigen/antibody pairs. The magnetic particles are then detected with a magnetic detection apparatus, thereby providing an indication of the concentration of the analyte of interest (e.g., the antigen or the antibody). By tagging analytes with magnetic nanoparticles, the problem of biological detection is in effect reduced to one of magnetic field measurement.
One magnetic immunoassay method involves scanning a giant magnetoresistance (GMR) sensor at a relatively large distance of several microns above a biological test sample. (See, for example, J. Nordling et al., “Giant Magnetoresistance Sensors. 1. Internally Calibrated Readout of Scanned Magnetic Arrays,” Anal. Chem., 80 (21), pp. 7930-7939, 2008; and R. L. Millen et al., “Giant Magnetoresistive Sensors. 2. Detection of Biorecognition Events at Self-Referencing and Magnetically Tagged Arrays,” Anal. Chem., 80 (21), pp. 7940-7946, 2008.) With this method, a relatively large distance between the sensor and the test sample is required, since the sample is typically fragile and would be easily damaged by the sensor. As a result, the magnetic particles must be correspondingly large to produce a sufficiently strong magnetic field at this distance. Accordingly, the spatial resolution that can be achieved is relatively poor. Also, large magnetic particles may require a greater number of analytes to join them to a functionalized sample surface, thereby decreasing the detection sensitivity.
In another magnetic immunoassay method, the analytes and magnetic particles are located directly on the GMR sensors' surface. (See, for example, G. Li et al., “Detection of single micron-sized magnetic bead and magnetic nanoparticles using spin valve sensors for biological applications,” Journal of Applied Physics, vol. 93 (10), 2003; and U.S. Pat. No. 7,682,838 to Wang et al. titled “Magnetic Nanoparticles, Magnetic Detector Arrays, and Methods for their Use in Detecting Biological Molecules”.) With a dedicated sensor being used for each test site and the magnetic particles being located at the sensor surface, the field sensitivity is quite high. On the other hand, this means that to process a large number of different analytes, it is necessary to have a very complicated test chip that includes a correspondingly large number of GMR sensors dedicated to respective test sites and analytes.